1. Field of the Invention
The invention relates generally to the field of weed management. More specifically, the invention relates to methods for pre-emergent and post-emergent herbicide use for controlling weeds in combination with transgenic crops tolerant to one or more herbicides.
2. Description of the Related Art
Weeds cost farmers billions of dollars annually in crop losses and the expense of efforts to keep weeds under control. Weeds also serve as hosts for crop diseases and insect pests. The losses caused by weeds in agricultural production environments include decreases in crop yield, reduced crop quality, increased irrigation costs, increased harvesting costs, decreased land value, injury to livestock, and crop damage from insects and diseases harbored by the weeds. The damage caused can be significant. For example, it is estimated that between 1972 and 1976 corn yields were reduced by about 10% due to weeds (Chandler, 1981).
Chemical herbicides have provided an effective method of weed control over the years. Herbicides can generally be applied pre-emergence and/or post-emergence. Pre-emergence herbicides are applied in a field before a crop emerges from the soil. Such applications are typically applied to the soil before or soon after planting the crop. Such applications may kill weeds that are growing in the field prior to the emergence of the crop, and may also prevent or reduce germination of weed seeds that are present in the soil. Post-emergence herbicides are typically used to kill weeds after a crop has emerged in the field. Such applications may kill weeds in the field and prevent or reduce future weed seed production and germination.
One weed control strategy is to apply an herbicide such as dicamba to a field before sowing seeds. However, after applying the herbicide to a field, a farmer has to wait at least several weeks before sowing the field with crop seeds such that the herbicide has killed most of the weeds and has degraded so as not injure the sown crop. For example, plants are especially sensitive to dicamba and it has been recommended that dicamba formulations such as Banvel™, Clarity™, or Sterling™ be applied, for instance, 30 days prior to planting for controlling weeds.
Another method that has been successfully used to manage weeds combines herbicide treatments with crops that are tolerant to the herbicide. In this manner, a herbicide that would normally injure a crop can be applied before and during growth of the crop without causing damage. Thus, weeds may be effectively controlled and new weed control options are made available to the grower.
In recent years, the availability of transgenic crops having traits providing tolerance to a herbicide or herbicides with one mode of action has simplified weed management for growers. For example, crops tolerant to 2,4-dichlorophenoxyacetic acid (Streber and Willmitzer, 1989), bromoxynil (Stalker et al., 1988), glyphosate (Comai et al., 1985) and phosphinothricin (De Block et al., 1987) have been developed. However, this strategy has increased the possibility of selection for and spread of weed biotypes resistant to a particular herbicide in a particular cropping system. Therefore, there is a need in the art for inventing cropping systems that use transgenic crops providing tolerance to one or more herbicides for managing weeds i.e., for managing current herbicide resistant weeds, for managing tough weeds, for managing volunteer plants, and for minimizing the development of herbicide resistant weeds in the future.
It is also known in the art that the risk of developing resistant weeds is higher with certain types of herbicides and lower with certain other types. For the following discussion, herbicides are classified according to their modes-of-action based on the HRAC or WSSA schemes (Table 2). For example, the risk of developing resistant weeds is thought to be higher with herbicides belonging to groups such as acetolactate synthase (ALS) inhibitors (Group 2 or B) and acetyl CoA carboxylase (ACCase) inhibitors (Group 1 or A). The risk of developing resistant weeds is thought to be lower with herbicides belonging to groups such as PS II inhibitors (Group 5 or C1), microtubule assembly inhibitors (Group 3 or K1), and lipid synthesis inhibitors (Group or N). The risk of developing resistant weeds is thought to be still lower with herbicides belonging to groups such as synthetic auxins (Group 4 or 0), glycines (Group 9 or G), and inhibitors of glutamine synthetase (Group 10 or H) (Légère et al., 2006.). Hence it is desirable to develop cropping systems utilizing crops tolerant to low-risk herbicides and their accompanying herbicide treatments for minimizing populations of herbicide resistant weeds.
Dicamba is one member of a class of herbicides commonly referred to as “auxin-like” herbicides or “synthetic auxins.” Dicamba has been used as a pre-emergence herbicide (e.g. 14-30 days prior to planting) in dicots and as a pre- and/or post-emergence herbicide in corn, sorghum, small grains, pasture, hay, rangeland, sugarcane, asparagus, turf, and grass seed crops to effectively control annual and perennial broadleaf weeds and several grassy weeds (Crop Protection Chemicals Reference, 1995). Unfortunately, dicamba can injure many commercial crops including beans, soybeans, cotton, peas, potatoes, sunflowers, tomatoes, tobacco, and fruit trees, ornamental plants and trees, and other broadleaf plants when it comes into contact with them. Soybean and cotton are particularly sensitive to dicamba. Thus, applications of dicamba must generally occur several weeks before planting of sensitive crops to ensure that residual dicamba is sufficiently cleared from the crop environment before crops emerge.
Recently, sequences encoding a multicomponent dicamba demethylase, including a monooxygenase (DMO), were isolated from Pseudomonas maltophilia (U.S. Patent Application Nos: 20030115626; 20030135879; U.S. Pat. No. 7,022,896) which is involved in the conversion of an herbicidal form of the herbicide dicamba (3,6-dichloro-o-anisic acid; a formulation of which is sold, for instance, under the trade name Banvel™) to a non-toxic 3,6-dichlorosalicylic acid (Wang et al., 1997). The inventors reported the transformation of the sequences into tobacco and Arabidopsis. The transformed plant tissue was selected on kanamycin and regenerated into a plant. However, herbicide tolerance was not demonstrated or suggested in immature tissues or seedlings or in other plants. Pre-emergence herbicide applications were also not described.
U.S. Pat. No. 6,376,754 describes plants, such as soybean plants, having tolerance to at least two herbicides. Included among these herbicides are glyphosate, glufosinate, and a sulfonylurea (i.e. an acetolactate synthase (ALS) inhibitor) herbicide. U.S. Pat. No. 6,586,367 describes methods to control weeds, and plants with tolerance to glyphosate or glufosinate, which may be treated with glyphosate or glufosinate, and additionally with an amount of an herbicide or herbicides selected from the group consisting of atrazine, dicamba, and other selected herbicides. However plants and cropping systems comprising a genetic trait conferring tolerance to dicamba are not described.
WO2005/107437 discloses combining a first herbicide tolerant gene i.e., a 2,4-D tolerance gene with a second herbicide tolerant gene i.e., a glyphosate tolerance gene or other herbicide tolerant gene. It does not disclose combining a glyphosate tolerant gene with a dicamba tolerant gene and a 2,4-D tolerant gene. Furthermore, it does not disclose cropping systems of the present invention for managing weeds, herbicide resistant weeds, tough to control weeds, herbicide resistant volunteer crop plants, and for minimizing the potential of herbicide resistant weeds in the future. It also does not disclose methods for minimizing development of herbicide resistant weeds in the future by rotating herbicide tolerant crops and use of their corresponding herbicide(s).